This invention relates generally to the vapor deposition of thin films and more particularly to a method of changing or modifying in situ the geometrical structure of a semiconductor device in epitaxial growth or during an interruption in epitaxial growth via chemical vapor deposition (CVD) and more specifically to a method of in situ, radiation assisted evaporation or desorption or vaporization (hereinafter referred to as "photo induced evaporation enhancement") of patterned or selected volumes of surface crystal material from a single element film (e.g., Si or Ge) or a binary, ternary and other compound semiconductor thin films, such as, II-VI or III-V compounds (e.g., ZnSe or GaAs) or alloys (e.g. Al.sub.x Ga.sub.1-x As), principally after their epitaxial crystal growth, but also during their epitaxial crystal growth, in vapor phase epitaxy (VPE) or metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE).
In U.S. patent application Ser. No. 07/177,563, filed Apr. 4, 1988, incorporated herein by reference thereto, there is disclosed a method of in situ stoichiometric and geometrical photo induced modification to compound semiconductor films. Disclosed in this application is the use of a scanned laser beam across or exposure of radiation through a mask to a growth surface to enhance the growth rate of epitaxial deposition in MOCVD. Variations in beam intensity, beam size, beam dwell time, or possibly changes in wavelength, if a tunable laser, via scanning or mask exposure, etc., at selected regions of the growth surface will selectively enhance the growth rate and/or in situ stoichiometric content during the deposition of a film on the growth surface. Such changes can produce individual films in situ having varying thicknesses and stoichiometric content useful in varying the electrical (e.g., bandgap or quantum size effect) and optical (e.g., refractive index or waveguiding) characteristics of the film in fabricating semiconductor devices.
As demonstrated in Ser. No. 07/177,563, changes in growth rate are accomplished by varying the growth surface temperature via changes in beam intensity or power density or substrate temperatures or combinations thereof. As is evident from FIG. 5 in Ser. No. 07/177,563, which illustrates the growth rate and photo induced growth rate enhancement of GaAs and AlGaAs as a function of substrate temperature, T.sub.s, changes in substrate temperatures at different growth surface locations can be brought about by different laser beam intensity exposures through a mask, or via beam scanning in relationship to dwell time, so that different growth rate enhancements can be achieved at different growth surface locations resulting in different film thicknesses or stoichiometric content at those different locations. In FIG. 5 of that application, laser enhanced growth of AlGaAs is observed to increase for substrate temperatures, T.sub.s, less than 610.degree. C. and the laser enhanced growth of the GaAs is observed to increase for substrate temperatures less than 565.degree. C.
To this extent, the process of that application provides a growth regime during in situ growth wherein different growth rate enhancements may occur at selected growth surface locations during film deposition due to different temperatures of operation thereby producing different locations in the same film having different film thicknesses. A specific example of this is represented in the data of FIG. 5 of that application wherein at one location of the growth surface, a smaller growth rate enhancement is achieved due to lower temperatures of operation, e.g., the absence of laser illumination over substrate temperature, compared to another location of the growth surface wherein larger growth rate enhancement is achieved due to higher temperature of operation, e.g. the presence of laser illumination in addition to substrate temperature at a selected growth surface location. The first mentioned location, therefore, results in smaller film thickness, as well as possibly stoichiometric changes, compared to the second mentioned location.
The selective variance of growth rate enhancements in different regions of the epitaxially grown film, varying in situ the geometry, bandgap, refractive index properties and other electrical and optical characteristics of films is also possible, with sufficient optical intensity, at substrate temperatures greater than substrate temperatures illustrated for growth rate enhancement in FIG. 5 in patent application Ser. No. 07/177,563. This effect is made possible by the exponential dependence of evaporation rate with temperature and the concomitant limiting effect upon growth rate. For the particular example in the case of GaAs, see FIG. 2 in the article of D. H. Reep et al, "Electrical Properties of Organometallic Chemical Vapor Deposited GaAs Epitaxial Layers", Journal of the Electrochemical Society, Vol. 131(11), pp. 2697-2702, Nov. 1984.
It is a principal object of this invention to provide such a desorption regime wherein the rate of evaporation of deposited constituents is greatly enhanced either during the growth of the film to decrease the effective growth rate or after film growth to provide, in either case, locations in the film of reduced thickness compared to other regions of the film. This desorption regime is termed herein as "photo induced evaporation enhancement".
Photo induced evaporation processes known in the art for the removal of material may be generally classified as either photothermal or photochemical evaporation. An example of photothermal evaporation is found in U.S. Pat. No. 4,388,517 wherein patterning can be achieved in a deposited metal overlayer by employing therebeneath an insulating underlayer of low thermal conductivity that is patterned prior to the metal overlayer deposition to reveal or expose regions of a base film therebeneath of high thermal conductivity. The exposure to high intensity radiation, such as a laser beam, will cause sublimation of the deposited metal overlayer in deposited regions over low thermal conductivity while those regions of the metal overlayer over patterned high thermal conductivity regions of the base layer will remain intact thereby forming a metallized overlayer pattern matching that formed in insulating underlayer. Large optical power densities (0.5 to 5 joules/cm.sup.2) are employed in order to effectively remove metal layers 10 nm thick and, in essence, entails a process of mask making rather than a process of patterned negative crystal growth.
Furthermore, studies have been conducted relative to the thermal evaporation of compound semiconductors, such as the congruent evaporation of GaAs, under equilibrium and nonequilibrium conditions. See, as an example, C. T. Foxon et al, "The Evaporation of GaAs Under Equilibrium and Nonequilibrium Conditions Using a Modulated Beam Technique, Journal of Physical Chemistry and Solids, Vol. 34, pp. 1693-1701 (1973). A decrease in the net growth rate of GaAs in MBE above 640.degree. C. was observed by R. Fischer et al, "Incorporation Rates of Gallium and Aluminum on GaAs During Molecular Beam Epitaxy at High Substrate Temperatures", Journal of Applied Physics, Vol. 54(5), pp. 2508-2510, May, 1983, which was attributed to Ga evaporation. Also, those knowledgeable in the area of thermal evaporation have studied the layer-by-layer growth and desorption of GaAs and AlGaAs observing that the growth rates as well as the sublimation or evaporation rates for these compounds in a MBE high vacuum system is a function of substrate temperature and the impinging arsenic flux. See the articles of J. M. Van Hove et al, "Mass-Action Control of AlGaAs and GaAs Growth in Molecular Beam Epitaxy", Applied Physics Letters, Vol. 47(7), pp. 726-728, Oct. 1, 1985 and T. Kojima et al, "Layer-By-Layer sublimation Observed By Reflection High-Energy Electron Diffraction Intensity Oscillation in a Molecular Beam Epitaxy System", Applied Physics Letters, Vol. 47(3), pp. 286-288, Aug. 1, 1985. The results were in quantitative agreement with the mass-action analysis of R. Heckingbottom, "Thermodynamic Aspects of Molecular Beam Epitaxy: High Temperature Growth in the GaAs/Ga.sub.1-x Al.sub.x As System", Journal of Vacuum Science and Technology B, Vol. 3(2), pp. 572-575, Mar./Apr., 1985.
Further, techniques in MBE processing using thermal evaporation have been employed to provide a pattern in heterostructures. In one case, a plurality of GaAs quantum well layers separated by AlGaAs barrier layers were grown in MBE on a GaAs substrate mounted on a slotted susceptor so that a temperature differential is established across the supported substrate. In this manner, the thickness of the deposited GaAs and AlGaAs layers would be thinner over deposited regions on firm substrate having a 30.degree. C. to 50.degree. C. higher temperature gradient over substrate temperature compared to adjacent regions over susceptor recesses. See W. D. Goodhue et al, "Planar Quantum wells With Spatially Dependent thicknesses and Al Content", Journal of Vacuum Science and Technology B, Vol. 6(3), pp. 846-849, May/June 1988. It was recognized that quantum well structures grown above 700.degree. C., the thickness of these alternating well/barrier layers decreases as the temperature increases.
In the other case, represented by two examples, patterning is achieved by quasi-in situ thermal processing wherein thermal etching is employed to selectively remove GaAs. In one example, a n-GaAs layer over a p-AlGaAs layer is initially, selectively chemically etched in a particular region followed by thermal etching to remove the remaining thin GaAs left from chemical etching before proceeding with regrowth of the p-AlGaAs layer. This forms a buried reverse biased current confinement mechanism in a double heterostructure laser. H. Tanaka et al, "Single-Longitudinal-Mode Self Aligned AlGa(As) Double-Heterostructure Lasers Fabricated by Molecular Beam Epitaxy", Japanese Journal of Applied Physics, Vol. 24, pp. L89-L90, 1985. In the other example, a GaAs/AlGaAs heterostructure partially masked by a metallic film is thermally etched in an anisotropic manner illustrating submicron capabilities for device fabrication. A. C. Warren et al, "Masked, Anisotropic Thermal Etching and Regrowth for In Situ Patterning of Compound Semiconductors", Applied Physics Letters, Vol. 51(22), pp. 1818-1820, Nov. 30, 1987. In both of these examples, AlGaAs masking layers are recognized as an etch stop to provide for the desired geometric configuration in thermally etched GaAs, although it is also known that, given the proper desorption parameters, AlGaAs may also be thermally etched at higher temperatures and different attending ambient conditions vis a vis GaAs.
However, none of these evaporation/desorption techniques employ photo induced evaporation as a technique in a film deposition system to incrementally reduce, on a minute scale, film thickness in patterned or selective locations at the growth surface either during or after film growth, producing smooth sculptured surface morphology which is a principal objective of this invention.
As to photochemical processes, there is a large area of art relating to photochemical or photoetching techniques relating to the chemical or ablative removal of materials from film surfaces or regions referred to as "laser microchemical processing" and disclosed in the article of F. Micheli and I. W. Boyd, "Laser Microfabrication of Thin Films: Part Part Three", Optics and Laser Technology, Part Three: Vol. 19(2), pp. 75-82, Apr., 1987. One such process referred to in this article is called ablative photodecomposition (APD) and is detailed in the article of R. Srinivasan entitled, "Kinetics of the Ablative Photodecomposition of Organic Polymers in the Far Ultraviolet (193 nm)", Journal of Vacuum Science and Technology B, Vol. 1(4), 923-926, Oct./Dec., 1983. Such photochemical and ablative type processes should not be confused with the invention herein, as this invention deals predominantly with a process that photothermally desorbs or evaporates material from a film surface as opposed to processes referenced above that deal predominantly with photochemical or photo-ablative removal of material from a film surface. While the process herein may possibly involve some photochemical efforts, the predominant effect in the execution of the method of this invention is photothermal evaporation.
Other objects of this invention are to bring about in situ evaporation of selected surface regions or layers of compound semiconductors without breaking the growth system environment employing photo induced evaporation enhancement in chemical vapor deposition epitaxy and, further, to apply this method in the fabrication of semiconductor devices, such as multiple wavelength light emitting semiconductor lasers or laser arrays, buried heterojunction lasers and laser arrays, and laser devices with buried back biased junctions for current confinement as further disclosed in U.S. patent application Ser. No. 07/328,988 and devices with buried impurity induced disordering sources as disclosed in U.S. patent application Ser. No. 07/328,275.